A typical Stirling cycle cryogenic refrigerator includes a compressor with a reciprocating piston. The reciprocating piston provides a near sinusoidal variation in the pressure of a working fluid. This working fluid fills a volume which includes the head space above the compressor piston and internal voids in a refrigerator cold finger. In the case of a split Stirling system, the working volume also includes a gas line interconnecting the compressor and the cold finger. Within the cold finger, a displacer moves in timed relationship with the sinusoidal pressure variations of the working fluid to provide cooling at one end of the cold finger.
Typically, Stirling cycle refrigerators include an electric motor driven compressor, and the invention will be described with reference to split Stirling cycle refrigerators of that type. In prior art compressors, the dynamic seal about the compressor piston has generally been an annular lip or a split ring seal.
One problem encountered when such compressors are used in split Stirling systems is leakage, past the dynamic seal, between the reciprocating piston and its surrounding housing. This leakage results in gas flow between the working volume at the headspace of the piston and a control volume or dead space to the rear of the piston. Where there is greater leakage in one direction than in the other during reciprocation of the piston, the mean pressure of the working fluid tends to rise or fall. Typically, refrigerators are designed about a certain designated mean pressure of working fluid. Therefore, as the seals deteriorate with compressor usage and the mean pressure of working fluid changes, a drop in refrigerator efficiency occurs. This drop in efficiency results in high energy usage and lower refrigerator capacity.
More recently, compressors of this type have been designed with clearance seals which are more resistant to wear and therefore have a longer life than seal rings. Further, such clearance seals are less likely than seal rings to inject contaminates into the working fluid that might degrade refrigerator performance. An example of this type of seal may be found in U.S. Pat. No. 4,539,818 issued on Sept. 10, 1985 to Noel Holland.
The compressor piston is generally driven by a connecting rod which is connected to the electrical motor. This connecting rod generally oscillates about 10.degree. with each rotation of the motor and reciprocation of the piston. A bearing therefore needs to be positioned between the connecting rod and the piston to allow for this oscillatory motion. In most instances, a ball or needle bearing is positioned around a wrist pin, which supports the piston. Such bearings need to be able to withstand extensive temperature changes and must operate in a substantially sealed environment so that bearing lubricants will not contaminate the working fluid.